1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus equipped with this sheet feeding apparatus, and particularly to a configuration for separating sheets that tend to stick to each other and feeding individual sheets.
2. Related Background Art
A conventional image forming apparatus, such as a copier or a printer, includes a sheet feeding apparatus wherein sheets stacked on sheet stacking means are sequentially fed, beginning with the uppermost sheet, by sheet feeding means, which is a pickup roller, and are separated by a sheet separating portion and individually supplied to an image forming portion.
Cut sheets, generally of high-quality paper or of a standard paper designated by a copier maker, are employed for the sequential feeding performed by a thus arranged sheet feeding apparatus. And to steadily separate and feed cut sheets individually, various sheet separating systems have been employed, such as a sheet separating pad system that, to prevent the double feeding of sheets, brings a friction member into contact with a feed roller.
As another separating system, there is a retard separating system wherein a separating portion is constituted by a feed roller, which is rotated in a sheet conveying direction, and a separating roller, which is driven at a predetermined torque in a direction opposite to the sheet conveying direction and which contacts the feed roller under a predetermined pressure, and wherein the separating portion passes only the uppermost sheet of a stack of sheets that is fed by a pickup roller, and returns, toward the sheet mounting means, other sheets that accompany the uppermost sheet, so that double feeding is prevented.
When one of the sheet separating systems, such as a retard separating system, is employed to steadily separate and individually feed sheets, a return torque and a pressurization force for a separation roller are optimized while taking the friction force of a sheet into account.
Recently, as the variety of types of sheets (recording media) has increased, the demand has likewise increased for the forming of images not only on very thick paper, OHP sheets and art films, but also on coated sheets, for which a surface coating process has been performed to obtain white and glossy colors that satisfy market demands for color.
However, when very thick paper is to be fed, it can not be picked up because the weight of the paper resists its conveyance, and a paper jam occurs. Further, when resin sheets, such as OHP sheets and art films, that tend to acquire a charge are to be fed in a low relative humidity environment, the surfaces of the sheets are gradually charged by rubbing against other sheets, and a Coulomb force causes then to attract to each other. As a result, either a sheet cannot be picked up, or the double feeding of sheets occurs.
Furthermore, a property of coated sheets the surfaces of which are covered with a coating material, is that when stacked they attract to each other, especially in a high relative humidity environment. Therefore, the coated sheets cannot be picked up individually, and the double feeding of sheets occurs.
The friction force exerted between the special sheets described above is equal to or smaller than the friction force for standard paper. However, in a low relative humidity environment, the attraction of resin sheets to each other is induced by an attractive force considerably stronger than the force generated by friction, and in a high relative humidity environment, the attraction of coated sheets to each other is induced by another attractive force that is considerably higher than the friction force. Therefore, the conventional separation system cannot perform individual sheet separation.
That is, since for the conventional sheet separation system only the friction force exerted between sheets is considered, this system cannot steadily separate individual sheets when an attractive force other than the friction force acts on sheets.
In order to eliminate the very high attractive force exerted between the sheets, conventionally, the printing industry and some copier manufacturers have adopted a sheet separation and feeding system as disclosed in Japanese Patent Application Laid-Open No. H11-005643. According to this system, individual sheets are raveled out in advance by blowing air against the side of a stack of sheets to remove attractions between sheets. In this state, the individual sheets are picked up, start with the uppermost, and are separated by a sheet separating portion located downstream. In the sheet separation and feeding system that comprises means (hereinafter referred to auxiliary raveling-out means) for blowing air against the side of a stack of sheets, the sheets (recording media) that tend to attract to each other are raveled out before the sheet feeding, and the attractions removed. Therefore, the efficiency of the sheet separation function is increased compared with the previously described system that relies only on the friction force. FIG. 18 is a diagram showing the configuration of a sheet feeding apparatus that includes such auxiliary raveling-out means. A sheet feeding apparatus 155 comprises: a sheet supply tray 59 on which sheets S are stacked; sheet feeding means (not shown), for feeding the sheets S from the sheet supply tray 59; air blowing means 71, for blowing air against the side of the stacked sheets S; and flow path moving means 157, for vertically moving the air blowing means 71 along the side of the stack of sheets S.
The flow path moving means 157 includes a guide rail (not shown), used to support the air blowing means 71 so it is movable vertically; an electric motor 121; and a cam plate 123, which contacts the lower face of the air blowing means 71 and moves the air blowing means 71 vertically. In the flow path moving means 157, when the electric motor 121 is rotated, the air blowing means 71 is moved vertically by the cam plate 123, and accordingly, an air channel is moved vertically. Since the opening (air blowing port) of the air blowing means 71 has a constant predetermined opening dimension, the side of the sheet S is exposed at the opening as the air blowing means 71 is lowered. Then, the dimension of the opening is reduced, and the direction in which air is blown from the opening is narrowed. As a result, the sheets P are floated beginning with the uppermost sheet S, and the attraction between all the sheets S is removed. Another example sheet separating and feeding system for blowing air against the side of a stack of sheets is disclosed in Japanese Patent Application Laid-Open No. 2001-048366. According to this system, blown air is heated by a heater to remove humidity from the sheets P in order to reduce the attractive force between the sheets (coated sheets), especially in a high relative humidity environment.
However, for a sheet feeding apparatus that employs the sheet separation and feeding system for blowing air against the side of a sheet stack, when air is blown, especially in a low relative humidity environment, only part of the stacked sheets close to the air blowing port is dried.
When the sheets are only partially dried, the surface resistance on the sheet plane is uneven, and as a result, when a sheet is fed to the image forming portion of the image forming apparatus, this dry portion causes a transfer failure, and an image defect occurs. Especially for an electrophotographic system wherein the image forming portion employs an electrostatic charge to transfer a toner image to a sheet, since the transfer function is greatly affected by the surface resistance of the sheet, the uneven surface resistance causes an uneven image transfer, so that considerable image deterioration occurs and the obtained image is very unsatisfactory.